Tropical cyclones (TCs) are among the most destructive weather systems affecting East Asia, with northern China particularly vulnerable to their associated systems. In recent decades, a poleward migration in the average latitude of TCs has been observed. However, the understanding of changes in precipitation in northern China associated with this shift remains poor. Thus, we investigate the variability of northward-migrating TCs (NTCs) during 1960-2024, coupled with an in-depth analysis of their associated precipitation changes. Results reveal that precipitation induced by NTCs shows robust upward trends, with TC-induced total precipitation (TCP) and extreme precipitation (TCEP) increasing by 0.83 (p<0.05) and 0.63 mm per decade, respectively. Both the rates of TCP and TCEP decadal intensification are notably higher in the early 21(st) century compared to the entire study period. Seasonal analysis indicates that TCP is concentrated in the Bohai Bay during summer in the past 15 years (2010-2024), while in autumn, it is centered in northeastern China. Additionally, the upward trend of TCP over the Bohai Bay in the past 15 years is primarily driven by typhoons and super-typhoons, and that over northeastern China is driven by severe tropical storms and severe-typhoons. The pronounced amplification of TCP during 2010-2024 (compare to 2000-2009) is likely attributed to stronger NTCs, longer durations and slower translation speeds. These findings highlight the escalating hydrometeorological hazards posed by NTCs in northern China, particularly in northeastern China, where TCEP has greatly intensified. Addressing the aforementioned emerging features of TCP and TCEP is crucial for improving drought-flood forecasting, disaster mitigation, and adaptation in these regions.

Based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and the Global Precipitation Climatology Project (GPCP) data, this research examines the atmospheric anomalies associated with the interannual variability of the South China Sea (SCS) summer monsoon (SCSSM) onset, focusing particularly on its connection with positional changes of the subtropical westerly jet (SWJ) and thermal conditions over the eastern Tibetan Plateau (TP). The analysis reveals distinct difference of the circulation patterns between early and late monsoon onset years, marked by pronounced cyclonic anomalies with intensified rainfall during early onsets, contrasted by anticyclonic patterns and reduced precipitation during delayed onsets. A key finding demonstrates that early SCSSM onset years coincide with a southward displacement of the upper-level SWJ north of the SCS. This positional shift generates upper-level ageostrophic southerly winds over the SCS, establishing a dipole pattern of vertical motion. Upper-level divergence and low-level convergence happen over the SCS (south of the jet core), and upper-level convergence with low-level divergence occur over the Yangtze River basin (north of the jet). Such configuration amplifies the meridional circulation anomaly, enhancing ascending motions in low-latitude East Asia while strengthening subsidence in mid-latitude regions. The study also further shows that thermal anomalies over the eastern TP significantly affect SWJ positioning and subsequent monsoon onset timing. Positive heating anomalies initiate an upper-tropospheric anticyclone, triggering eastward-propagating Rossby waves and downstream cyclonic circulation. This prompts a southward migration of the SWJ east of the TP, altering East Asian circulation patterns to facilitate an earlier SCSSM establishment. These results shed new light on the TP's role in regional climate modulation via upper-level jet dynamics, offering potential predictive value for monsoon onset forecasting.

Significant winter [December-February (DJF)] precipitation over southern China (SC) is one of the key features of the East Asian winter monsoon, accounting for nearly 20% of annual precipitation in the area. While oceanic drivers of its interannual variability are extensively studied, the influence of atmospheric rivers (ARs), contributing approximately 30%-40% of the climatological wintertime precipitation in SC, remains unclear. Additionally, how seasonal forecast models capture the impact of tropical sea surface temperature (SST) variations on winter precipitation through ARs requires further investigation using objective metrics. This study identifies a tropical SST pattern involving El Ni & ntilde;o-Southern Oscillation (ENSO), the Indian Ocean dipole, and the SST anomalies over the western North Pacific (WNP), whose coevolving structure rapidly develops from the preceding summer to winter. This anomalous SST configuration generates a hemispheric-scale circulation pattern from the tropics to the subtropics, which enhances vertical wind shear and meridional moisture transport over SC, favoring increased AR intrusion into the region. Consequently, significant precipitation anomalies occur particularly near SC along 20 degrees-30 degrees N, explaining over 50% of the interannual DJF precipitation variability. These ENSO-driven precipitation changes, mediated by AR activity, are reasonably predicted by two operational seasonal forecast systems, suggesting that ENSO and its interaction with WNP SST anomalies serve as the primary sources of forecast skill for winter ARs and SC precipitation. Furthermore, a screening scheme based on the observed SST and circulation states during October and November preceding the target winter is developed to determine the years in which the dynamical model forecast skill for SC DJF precipitation is higher than in other winters.

This study evaluates the simulation capabilities of 11 CMIP6 global climate models for hydrological cycle components in Northwest China (NWC) during 1995-2014, utilizing ERA5 reanalysis data for validation. Results demonstrate that the CMIP6 ensemble effectively captures spatiotemporal patterns of precipitation, precipitation recycling ratio (rho), and internal/external cycling precipitation. Subsequent analysis compares projected hydrological changes under SSP1-2.6 (low-emission) and SSP5-8.5 (high-emission) scenarios (2021-2100) against the historical baseline. The findings suggest that NWC is projected to experience a wetter climate in the future, with precipitation increase rates of 0.52 % per decade under the SSP1-2.6 and 3.12 % per decade under the SSP5-8.5. The most rapid intensification occurs in the early 21st century (2021-2040), followed by mid-21st century (2041-2060) deceleration in the growth rate. By the end of the century (2081-2100), precipitation declines under SSP1-2.6 but resurges under SSPP5-8.5, with maximum moistening concentrated in central NWC. Regarding the precipitation recycling rate (rho), the overall rate in NWC is projected to fluctuate within +/- 5 % under the SSP1-2.6. In contrast, it is expected to exhibit a fluctuating downward trend under the SSP5-8.5, decreasing by 1.04 % per decade and reaching nearly 5.1 % lower than the baseline period by the end of the 21st century. This indicates progressive weakening of internal cycling under high emissions. Spatially, the distribution of rho is highly uneven. The internal cycle is expected to increase significantly in the central region of NWC due to the abnormal enhancement of evaporation, while it will gradually weaken over time in other regions. Future precipitation increases primarily derive from the combined effects of internal and external cycling, but is primarily driven by changes in external cycling precipitation (P-o) through enhanced moisture transport. Moreover, under the high emission scenario, the contribution from external cycling demonstrates progressive increase, while internal cycling exhibits gradually weaken. This suggests that as climate warming accelerates in NWC, the rapid ablation of glaciers and snowmelt may lead to a transition from strengthening to weakening of the internal cycle because of the increasing ineffective evaporation. Even if the absolute precipitation increases in the future, the additional moisture may be offset by ineffective evaporation, making it difficult to convert into usable water resources. This poses a severe challenge for future water resource management in NWC.

The Meiyu-Baiu-Changma (MBC) is a critical rainy season in East Asia. The MBC rainfall is a vital water source but also causes devastating flooding, profoundly impacting agriculture, water resource management, and socio-economy across East Asia. The El Niño–Southern Oscillation (ENSO) plays a critical role in modulating the interannual variability of MBC. The response of MBC to ENSO is, however, complex, nonlinear, and stochastic, influenced by various ENSO characteristics including the phase, intensity, location, and decay pace. This review synthesizes recent advances in understanding the ENSO–MBC linkage, by incorporating existing literature and our new analyses, to elucidate the underlying mechanisms, model performance, and future projections regarding ENSO's impacts on the MBC under climate change. In this review, an increased correlation between ENSO and MBC over past decades is revealed. The two main paths of ENSO impacting the MBC via modulating the anomalous western North Pacific anticyclone, and the changes in the influence of these paths under climate change, are synthesized and analyzed. Seasonal prediction of ENSO-driven MBC anomalies remains challenging, despite the advances of climate models in simulating and predicting the ENSO-related large-scale ocean and atmospheric circulation anomalies. In the future, intensified global warming may lead to a further strengthened impact of ENSO on MBC and increased ENSO-driven MBC extremes. Exploring greenhouse gas forcing's influence, improving high-resolution coupled models, refining representation of key dynamic processes, and utilizing artificial intelligence techniques are essential to advance understanding, simulation, prediction, and climate adaptation strategies related to ENSO-MBC connection.

This study explores the impact of winter sea surface temperature(SST) anomalies in the Southern Indian Ocean on summer precipitation patterns in China, utilizing data from reanalysis sources and Coupled Model Intercomparison Project Phase 6(CMIP6) models. The results reveal that the Southern Indian Ocean Dipole(SIOD), characterized by contrasting SST anomalies in the northeast and southwest regions, acts as a predictor for Chinese summer precipitation patterns, namely floods in the south and drought in the north. In a positive SIOD event, the southwestern Indian Ocean exhibits warmer SSTs, while the northeastern region remains cooler. A negative SIOD event shows the opposite pattern.During the positive phase of the SIOD, the winter SST distribution strengthens the 850-hPa cross-equatorial airflow,generating a robust low-level westerly jet that enhances water vapor transport to the Bay of Bengal(BoB). These air-sea interactions maintain lower SSTs in the northeastern region, which significantly increase the land-sea temperature contrast in the Northern Hemisphere during spring and summer. This strengthened thermal gradient intensifies the southwest monsoon, establishing a strong convergence zone near the South China Sea and amplifying monsoon-driven precipitation in South China. Additionally, CMIP6 models, such as NorESM2-LM and NorCPM1, which accurately simulate the SIOD pattern, effectively capture the seasonal response of cross-equatorial airflow driven by SST anomalies of Southern Indian Ocean. The result highlights the essential role of cross-equatorial airflow generated by the SIOD in forecasting crossseasonal precipitation patterns.

The sea surface temperature (SST) warming in the high-impact area of the North Atlantic prompts active convection over the Qinghai–Tibet Plateau (QTP), which consequently drives the Hadley Cell (HC) in the South Asian monsoon region to shift northward. This interaction mechanism stresses the “hub” effect of the QTP in the atmospheric energy and water cycle of the low- to mid–high latitude systems during the convergence of westerly and monsoon winds. The Rossby source, also famous as the “oscillation source,” formed in the upper troposphere by the SST variations in the high-impact area of the North Atlantic, is an essential “thermal driving source” for the interannual shifts in convection over the QTP. The meridional teleconnection wave train structure triggered by the warming (1991–2020)/cooling (1961–1990) of the SST in the high-impact area of the mid–high latitudes of the North Atlantic displays a reversed phase. The Rossby wave train, which spreads from the North Atlantic to the QTP during the high-impact sea surface warming phase in the North Atlantic, indicates a remarkable anticyclonic structure (strong divergence) in the high altitude (200 hPa) of the QTP, which favors the generation of active convective activity in the latter 30 years. By contrast, convective activity is blocked. During the two stages of 1961–1990 and 1991–2020, despite a significant interdecadal positive and negative phase reversal in the North Atlantic Multiyear Oscillation (AMO), the variance in the definition range between the AMO and the high-impact area of the North Atlantic led to substantial differences in the meridional teleconnection wave train structures and the corresponding effects. In addition, the latent heat emitted by the enhanced convective activity on the QTP during the sea surface warming phase in the high-impact area of the North Atlantic can strengthen the “heat pump” effect of the QTP, cause the northward shift of HC in the South Asian monsoon region, and spark the mutual feedback mechanism between the plateau convection and the HC in the South Asian monsoon region. According to these interdecadal response characteristics, this paper offers a comprehensive physical image that exhibits the mutual feedback between the convection over the QTP and the HC in the South Asian monsoon region, where the active convection is initiated by the SST warming in the high-impact area of the North Atlantic.

Global warming has increased the frequency of compound extreme heat events in China, posing significant health risks. Pacific sea surface temperature (SST) signals are crucial for the occurrence of extreme heat events, yet previous research has not clearly differentiated their contributions at various timescales to compound heat events with high health risks (HRCHEs). Using low-frequency component analysis, this study quantifies the contributions of long-term, decadal, and interannual modes of winter pacific SSTs to high health risk dry-heat and wet-heat compound events (HRDHs and HRWHs) in summer across China on the basis of emergency medical and meteorological data. Results indicate that the contribution of Pacific SST signals to HRWHs is greater than that to HRDHs, with the total contribution to HRWHs ranging from 7 to 47

目前对复合极端事件的定义主要从气象和统计学角度出发,未能结合健康数据,使相关研究在指导健康风险预防方面的实用性较差.本研究利用中国死亡数据和温湿度数据,识别了中国高死亡风险的复合极端干热事件(HMHDs)的阈值,并探讨了HMHD频率的年代际变化及其驱动机制.结果显示,中国西北地区,西南地区和江南地区的HMHD频率平均每年都超过10天.2000年后,中国HMHD频率显著增加,主要发生在夏季.驱动机制分析表明,HMHD频率的年代际变化可以归因于大西洋多年代际振荡由负相位转变为正相位.本研究可为气候变化导致的健康风险防控提供参考依据.

Drought to flood abrupt alternation (DFAA) events, as a special category of compound extreme events that suddenly shift from drought to flood conditions, have significantly greater impacts than individual drought or flood events. In this paper, we have utilized a multifactorial drought index and flood index to identify daily DFAA events occurring in mainland China and in major impact areas during the period 1961–2022. Based on drought and flood index, we have used entropy weighting method to measure the intensity of DFAA events. Our findings indicate that China's DFAA events primarily occur in the hotspots of Huang-Huai-Hai River Basin, the middle and lower Yangtze River Basin, the southeastern coastal area, and the southwestern part of the country. The most frequent and intense DFAA events occur from June to September, with varying subseasonal patterns in the frequency and intensity of events in each hotspot. The frequency of DFAA events in mainland China shows a significant decreasing trend declining at a rate of 0.25 per year in year-round. While DFAA events occurring in the warm season tend to decrease more significantly compared to the year-round at a rate of 0.26 per year. However, the intensity of DFAA events is increasing with a rate of 0.1 per decade in both the year-round and warm season. The evolution of DFAA events and their direct causes varies non-uniformly across regions and months. Subseasonally, frequency and intensity trends diverged monthly across regions, notably with the Huang-Huai-Hai Basin and southeast coast experiencing a July decline in frequency but a surge in intensity. Our research deepens the understanding of changes in DFAA events and provides practical reference for preventing and mitigating drought-to-flood disasters in mainland China.

Frequent occurrences of compound droughts and heatwaves (CDHWs) may have considerable adverse impacts on human health and socioeconomic development. To project future summertime CDHWs over the Yangtze River Delta (YRD) of China under carbon neutrality (CN) for policymakers, we used nine global climate models (GCMs) from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and evaluated and projected circulation conditions tied to the summer YRD CDHW frequency in the boreal summer. The results show that a marked mid-tropospheric regional-scale anticyclonic anomaly centred over the East China Sea can be deemed the dominant system linking the more frequent occurrence of YRD CDHWs. Our selected nine CMIP6 GCMs better captured the observed climatological and anomalous spatial circulation patterns connected to the summer YRD CDHW frequency. Moreover, the nine models' multi-model ensemble mean (9MME) was better than most of the individual models regarding the related climatological circulation simulations. The 9MME-projected changes in circulation conditions tied to summer YRD CDHWs during the carbon neutrality period (2050-2060) under SSP119 favour the comparable occurrence of the YRD with that for the same period under the medium greenhouse gas (GHG) warming scenario (SSP245), but the intensities of the CDHWs are the least severe. However, the highest number of YRD CDHWs is driven by the high GHG warming scenario (SSP585), and these CDHWs feature the most severe intensity. Our research identified a new paradigm regarding the frequency and intensity of the summer YRD CDHW under SSP119, highlighting the potential role of the newly accessible CN-oriented sustainable development pathway in alleviating compound extremes.

Geopotential height ( H ) is a widely used metric for atmospheric circulation. H has been reported to rise under global warming, but the amplitude and mechanism of this rise are not clear. Based on reanalysis datasets and climate models participating in CMIP6, this study quantitatively evaluates the sensitivity of H to global mean surface air temperature ( T s ), i.e., d H /d T s . Reanalysis datasets and model simulations consistently show that d H /d T s increases monotonically with altitude in the troposphere, with a global averaged value of about 24.5 gpm/K at 500 hPa, which overwhelms the interannual H variability. Diagnosis based on the hypsometric equation shows that the rise in global H is dominated by expansion of the air column due to warming-induced reduction in air density, and the magnitude of d H /d T s is determined largely by a vertical integration of the warming profile below the pressure level. Since the anthropogenic forced rise in H is rather horizontally uniform and proportional to T s change, past and projected future changes in the global H field at each pressure level can be reproduced by change in T s multiplied by a constant historical d H /d T s value. Spatially uniform rise in H reproduces the past and projected future expansion of the widely used H = 5880 gpm contour at 500 hPa, suggesting that it does not indicate enhancement of the subtropical high but is simply caused by thermal expansion of the atmosphere. This work uncovers the physical mechanism for rising H and offers a simple way to estimate H anomaly based on T s anomaly.

Based on ERA5 and 23 CMIP6 models under three Shared Socioeconomic Pathways (SSPs), we assess the performance of CMIP6 models in reproducing the summer precipitation, evaporation and precipitation recycling ratio (PRR) over the Tibetan Plateau (TP) during 1981-2014. We also project future changes of the summer precipitation, evaporation, PRR and moisture transport over the TP during 2021-2100. CMIP6 models and their ensemble mean reproduce the temporal and spatial characteristics of precipitation, evaporation, PRR and moisture transport over the TP. And the performance in simulating precipitation is better than that in simulating evaporation and PRR. The precipitation and evaporation under the three Shared Socioeconomic Pathways (SSPs) will increase, with the significant upward trends of 0.84%/decade~2.79%/decade and 0.80%/decade~2.14%/decade, respectively. However, the PRR will increase slightly (0.13%/decade) then marginally reduce (-0.22%/decade). In the late 21st century, regional mean summer precipitation, evaporation and PRR over the TP in SSP5-8.5 will change by about 20.13%, 17.14% and -0.77%, respectively. From spatial distribution, precipitation will increase in most parts of the TP in the 21st century, especially in the western part of the plateau, reaching more than 50% in of the late 21st century, the area of PRR decreased gradually expanded, with the maximum reduction (15~20%) of PRR in the western region. It indicates that more precipitation in the western of the plateau depends on the external moisture transport. The PRR in the southern part of the plateau will increase, with a maximum of more than 20%, which indicates the contribution of the external water vapor transport to the southern precipitation will decrease, and that of the internal cycle (local evaporation) to the local precipitation will increase. In conclusion, the increase of the regional mean precipitation over the TP in the near and mid-21st century is mainly due to the local evaporation, while the increase of the summer precipitation in the late 21st century is due to both the local evaporation and the external water vapor advection, it shows that the precipitation generated by the inner circulation is becoming weaker and more dependent on the external water vapor transport.

Over Northeast China, the area surrounding the Horqin sandy land (HSL) is an important plantation domain within the “Three north” Area of China. Compound heat waves and droughts can exert exceptionally negative impacts on ecosystems. However, less attention has been paid to the physical causes of the variability of the frequency compound droughts and heatwaves in this peculiar sandy region. In this study, salient climatic factors tied to the variability of the number of compound drought and heatwave day (CDHD) around HSL in late summer (July–August) are identified. Results show that the preceding wintertime El Niño–Southern Oscillation (ENSO)-associated SST mode can exert significant delayed impacts on the interannual CDHD fluctuations around HSL in the following late summer, serving as a precursory oceanic factor. Dynamical and thermodynamical diagnoses associated with the observational evidences and the Community Earth System Model version 2 (CESM2) tropical Pacific pacemaker experiment outputs reveal and confirm that the negative phase of ENSO (i.e. La Niña-associated sea surface temperature mode) in the prior winter can exert cross-season impacts on the enhanced CDHDs around HSL in the subsequent late summer through a critical synchronous atmospheric bridge of the western North Pacific cyclonic anomaly. This system may connect an anticyclonic anomaly prevailing over Northeast Asia via an anomalous meridional overturning circulation, which could act as a salient synchronous atmospheric factor. In this circumstance, favorable in situ meteorological and land surface conditions (e.g., increased 2-m maximum air temperature and suppressed rainfall/soil moisture deficiency) can be established, eventually sparking abnormal heat and drought conditions surrounding HSL and thus producing more late-summer CDHDs around HSL.

Based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and the Global Precipitation Climatology Project (GPCP) data, this study investigates the meteorological anomalies tied to the interannual variability of the South China Sea summer monsoon onset (SCSSMO), and its relationship with the change of the subtropical westerly jet position (SWJP), and the potential influence of the precursory thermal forcing over the eastern Tibetan Plateau (TP). The results show that during the earlier (later) SCSSMO years, there exist significant cyclonic (anticyclonic) circulation anomalies over the South China Sea (SCS) and its adjoining areas, featuring enhanced (suppressed) precipitation. The earlier SCSSMO years correspond to the southward-shifted upper-level subtropical westerly jet position to the north of the SCS. This favors the occurrence of non-geostrophic southward winds in the upper troposphere, the occurrence of upper-level divergence (convergence) and low-level convergence ascending (divergence) over the SCS and its nearby areas (Yangtze River basin) on the south (north) side of the jet axis, and the strengthening of the meridional circulation anomaly with anomalous ascending over the low-latitude and descending over the middle-latitude East Asia. Further analysis suggests that the anomalous heating over the eastern TP is significantly related to the SCSSMO and the change of SWJP. When the previous first-two-pentad heating anomalies over the eastern TP are positive, an anomalous anticyclonic circulation is formed in the upper troposphere, stimulating an eastward-propagating wave train. This, in turn, generates an anomalous cyclonic circulation downstream of the TP in the upper troposphere. As a result, the subtropical upper-level westerly jet downstream of TP shifts southward, which further affected the variation of atmospheric circulation over East Asia, ultimately leading to the earlier SCSSMO.

Based on long-term observational and reanalysis datasets from 1901 through 2014, this study investigates the characteristics and physical causes of the interdecadal variations in the summer precipitation over the East Asian monsoon boundary zone (EAMBZ), which is a peculiar domain defined from the perspective of the interplay between climatic systems (i.e., mid-latitude westerly and East Asian summer monsoon). Observational evidence reveals that, similarly to previous studies, the EAMBZ precipitation featured prominent interdecadal fluctuations, e.g., with dry summers during the periods preceding 1927, 1939–1945, 1968–1982, and 1998–2010 and wet summers during the periods of 1928–1938, 1946–1967, and 2011 onwards. Further analyses identify that, amongst the major interdecadal oceanic forcings (e.g., Atlantic multidecadal oscillation and Pacific decadal oscillation), the Indian Ocean basin mode (IOBM) is a significant oceanic forcing responsible for the interdecadal variations of the EAMBZ precipitation, playing an independent and critical modulation role. When the cold phase of the IOBM occurs, an anomalous cyclonic circulation is excited around the northeast corner of the tropical Indian Ocean, which further induces a north-low–south-high meridional seesaw pattern over the Northeast China–subtropical western Pacific (SWP) sector. Such seesaw pattern is conducive to the enhanced EAMBZ precipitation by linking favorable environments for the transportation of water vapor from the SWP and the convergence over the EAMBZ at interdecadal timescales. For this reason, a physical–empirical model for the EAMBZ precipitation is developed in terms of the IOBM cooling. Despite the fact that the extreme summer EAMBZ precipitation cannot be captured by this model, it can still well capture its interdecadal fluctuations and reflect their steady relationship. The key physical pathway connecting the IOBM cooling with the interdecadal variations of the summer EAMBZ precipitation is supported by the numerical results based on the large ensemble experiment and the Indian Ocean pacemaker experiment. Our findings may provide new insights into the understanding of the causes of the interdecadal variations in the summer EAMBZ precipitation, which may favor the long-term policy decision-making for the local hydrometeorological planning.

This study comprehensively analyzed the long-term changes of Meiyu in the middle and lower reaches of the Yangtze River with regard to precipitation, precipitation threshold, and interdecadal changes in atmospheric circulation. Results revealed obvious new features of Meiyu precipitation since 2000. (1) Meiyu precipitation shows a significant linear increase trend, with an average increment of 73.5 mm every 10 years. The increase in torrential rain is the most significant, accounting for 61% of the total precipitation increase, and its proportion has been increasing, while the proportions of light, moderate, and heavy rain in the total precipitation have been decreasing. (2) At the interdecadal scale, Meiyu precipitation and the local surface average temperature show opposite changes. Meiyu precipitation decreases by approximately 150 mm for every 1°C increase in the average temperature. This inverse correlation was not evident before 2000. (3) The saturated specific humidity in this area shows a significant increasing trend, indicating that precipitation in this area is caused by a higher threshold of atmospheric saturation and condensation, which may be a reason for the low Meiyu precipitation during high-temperature years. (4) The main atmospheric circulation in East Asia associated with Meiyu shows clear interdecadal changes, including the Western North Pacific Subtropical High and South Asian High having become significantly stronger and having extended westward and eastward, respectively, thereby facilitating the occurrence and persistence of Meiyu precipitation. The atmospheric circulation patterns associated with Meiyu, such as the Western North Pacific Subtropical High and South Asian High, have more significant impacts on Meiyu precipitation.

The impact of early winter (December) North Atlantic Oscillation (NAO) on the subsequent dramatic seesaw haze intensity alternation (DSHA) phenomenon in the North China Plain (NCP) between late winter months of January and February is identified based on observational analyses and Community Earth System Model Large Ensemble Numerical Simulation (CESM-LENS) datasets in this study. Our research suggests that the late winter abrupt subseasonal haze intensity enhancement is preceded by the negative NAO-like pattern in December, and vice versa. Detailed analysis indicates that persistent North Atlantic tripole (NAT) sea surface temperature (SST) patterns play a crucial oceanic bridging role in relaying the delayed influence of NAO from December to the following January. In this circumstance, negative December NAO can induce a profound planetary-scale zonal Rossby wavetrain emanating from the central North Atlantic, in which the southern branch of this wavetrain propagates the negative NAO signal downstream to northeastern Asia, forming the Northeast Asian cyclonic anomaly in January and facilitating the repressed haze intensity via inducing the haze-unfavorable meteorology. However, in the subsequent February, the cyclonic anomaly centered southeast of the Yamal Peninsula plays a critical role in prolonging the influence of the December NAO, acting as an intermediate atmospheric bridge effect. The notable Northeast Asian anticyclonic anomaly can be induced in February accordingly, thus facilitating the haze intensity enhancement in the NCP via inducing the haze-favorable meteorology. As such, the dramatic subseasonal haze enhancement in the NCP between late winter months can be triggered by the preceding early winter NAO. It shows that the early winter NAO could be considered as a crucial precursory atmospheric signal predicting the late winter DSHA phenomenon in the NCP.

The North China mid-summer (July) precipitation (NCJP) contributes the largest proportion of total annual precipitation in North China, with significant interdecadal and interannual variability. The interannual variability of the NCJP was further investigated on the basis of a study of its interdecadal variability and found that a sea surface temperature (SST) pattern in July located in the northwest Pacific, defined here as the northwest Pacific SST tripole (NWPT), can significantly influence the interannual variability of the NCJP, and that this relationship is regulated by the decadal northern North Atlantic SST (NNASST). Diagnostic analysis and the linear baroclinic model experiment indicate that the positive (negative) NWPT in July can excite an anomalous anticyclone (cyclone) in the region centered on the Korean peninsula and an anomalous cyclone (anticyclone) in the northwest Pacific off southeast Japan, thereby strengthening (weakening) the NCJP. When the decadal NNASST is in a significantly positive phase, the positive geopotential height anomalies it excites in the northwestern region off North China are not favorable for the connection between the NWPT and the NCJP. When the decadal NNASST is in a negative or insignificantly positive phase, the July NWPT and the NCJP have a significant positive correlation on interannual timescale.

The Northwest region is the major battlefield and important ecological and environmental security barrier area of China’s western development. Climate change in this region has a direct impact on water resources, ecology and environmental security in the Belt and Road Initiative strategy. In the context of global climate change, the climate in Northwest China has shown an obvious warming-wetting phenomenon with a rapid development trend, which has resulted in increasingly prominent environmental and public security risks, causing serious harm to the sustainable development of regional economy and society. This poses new risk challenges for climate change response, water resource management, and disaster prevention and mitigation in the region. The researches on the evolution characteristics, causes and physical mechanisms of Warming-Wetting as well as its future trends and possible risks were reviewed. The existing scientific consensus were summarized and the existing problems and shortcomings in current research were analyzed, and in the end the key directions of future scientific research were put forward.